1. Field of the Invention
The invention relates to solid polyvinyl ester and poly(meth)acrylate resins with high polydispersity, processes for preparing them, and their use.
2. Description of the Related Art
In the preparation of solvent-based adhesive compositions it is common to use solid polyvinyl acetate resins, on account of their excellent adhesion to a multiplicity of substrates (DE 20 2004 014 691 U1). DE 1 060 142 AS recommends using solid, low molecular weight polyvinyl acetate resins with a narrow molecular weight distribution to produce adhesives and paints. For that purpose the solid resin is prepared by the suspension polymerization process, with catalyst and regulator used in an equimolar ratio with respect to one another, and being metered in continuously together with the monomers in order to maintain a constant monomer concentration. In order to improve the solubility of solid vinyl acetate-ethylene copolymer resins in the organic solvents typical for adhesives, DE 1 745 570 OS recommends accompanying their preparation by the use of aliphatic aldehydes having a chain length of 4 to 8 C atoms.
One of the decisive factors affecting the strength of the adhesive bond (cohesion) is the molecular weight of the solid polyvinyl acetate resin employed. The higher the molecular weight, the higher the strength that can be achieved. The increase in the molecular weight of the resin, however, is paralleled, for otherwise identical polymer composition, by an increase in the viscosity of the resin solution. If the intention is to set a particular viscosity which is favourable for the application, in an adhesive based on a relatively high molecular weight resin, it is then necessary, for a given amount of resin, to employ a greater amount of solvent. Because of the emission of solvents as adhesives cure, however, the actual aim is to reduce the amount of solvent as far as is possible.